The present invention relates to a force balancing manometer for measuring differential pressure over a wide range.
There is a fast growing need for accurate measurement of pressures and differential pressures of pure gases as well as mixtures of gases in vacuum systems used in the manufacture of computer chips and similar high technology devices. So-called capacitance manometers because of their cleanliness and sensitivity are widely used for this purpose almost to the exclusion of all other types of devices.
In a capacitance manometer, a relatively thin diaphragm separates the gas in pressure chamber 1 from the gas in chamber 2. A pressure differential across the diaphragm causes it to be deflected until the deformation induced force is equal to the pressure induced force. A capacitance probe is used to determine the amount of deflection which may then be related to the pressure difference across the diaphragm.
Capacitance manometers suffer from a number of deficiencies. For example:
1. In order to measure small differential pressures, the diaphragm must be thin and flat so that the spring rate is very small. However, a thin flat diaphragm is easily damaged by excessive stresses caused by over pressure conditions. Various diaphragm support means are known but costly over pressure accidents still occur. Because the flat diaphragm deforms in a complex shape, effective stops are also relatively expensive to machine.
2. Because of the use of a thin flat diaphragm, capacitance manometers have limited pressure range, typically only four decades. Therefore, multiple manometer heads must be installed to provide pressure measurement over the 7 or 8 decades of pressure range typically required.
3. Because the diaphragm is purposely deformed during the measuring process, capacitance manometers are extremely sensitive to changes in material properties with temperature or age.
4. Capacitance manometers have sensitive electrical circuits exposed on one side of the diaphragm which are readily damaged by process gases.
5. Capacitance manometers require costly vacuum compatible electrical feedthroughs to conduct electrical signals through the vacuum wall.
6. Capacitance manometers suffer from hysteresis effects. The pressure indication at a given pressure depends on whether the given pressure is approached by increasing or decreasing the pressure.
7. Capacitance manometers require extreme care in manufacture to assure the thin diaphragm is flat and uniformly stressed and are therefore very costly to manufacture.
Clearly what is required is a manometer which has a wide range, and is immune to overpressure, hysteresis and changes in material properties.
Mechanical Deformation Manometers
The above cited problems of capacitance manometers are largely caused by the use of the diaphragm to provide a mechanical deformation force to oppose the pressure induced force. The same type of problems are inherent in the design disclosed in Soviet Union Patent RU2010201, wherein the opposing force is provided by dual flexible bellows. Although using a deformed flexible member for the function of providing the opposing force is simple in principle, it causes many serious problems in practice. Similar problems are inherent in the design disclosed in U.S. Pat. No. 5,457,999, wherein the opposing force is provided by deforming an elastic vibrating member in tension.
Force Balancing Manometers
Manometers are known which do not depend on mechanical deformation of a flexible member for generating a balancing force to oppose the pressure induced force. These so-called force balancing manometers may be divided into two classes; those which measure a single pressure differential and those which measure multiple pressure differentials.
An example of the first class is disclosed in U.S. Pat. No. 3,657,630 wherein it is proposed to use a relatively large dc current in a flat diaphragm which is immersed in a magnetic field parallel to the diaphragm to provide an opposing force perpendicular to the diaphragm. This design depends critically on providing and maintaining a uniform current density across the diaphragm. Such a requirement is very difficult to achieve in practice and to our knowledge this invention has not found significant use.
An example of the second class is disclosed in U.S. Pat. No. 3,832,618 wherein two pressure differentials with a common pressure are measured and their difference determined. Thus, a third pressure differential is determined and displayed. The common pressure is produced by hydraulic fluid which is throttled and used for damping purposes. This patent for measuring very high pressure differentials does not teach how to accurately measure the very low pressures commonly used in vacuum processing and still achieve wide range. In U.S. Pat. No. 5,457,999 two pressure differentials with a common very low pressure are measured and their difference determined as noted above. However, this design does not employ force balancing and thus suffers from the effects of changes in material properties as noted above under mechanical deformation manometers.
Existing force balancing manometers such as are cited above are seldom if ever used in clean low pressure vacuum processing. This is likely because of lack of stability, inadequate sensitivity and the complexity and the high cost of providing force balancing means in vacuum.
The prior art for measuring multiple pressure differentials does not teach how to simultaneously avoid overpressure problems and still obtain high sensitivity. The prior art also does not teach how to avoid the serious problems of the influence of material property changes on accuracy.
The present invention relates to a force balancing manometer for measuring a differential fluid pressure. The fluid to be measured may be a gas or a liquid, the reference pressure medium may be a like or differing fluid.
The differential pressure is applied to a displaceable force sensing assembly and deviations of the force sensing assembly from a null position are sensed. A servosystem controls a force balancing means acting on the force sensing assembly so as to return the force sensing assembly to a null position. The force required to restore the force sensing assembly to the null position is calibrated to correspond to the differential pressure acting on the force sensing assembly.
The present invention provides a flexibly suspended circular first plate to isolate a first chamber at pressure P1 from the atmosphere and a flexibly suspended circular second plate to isolate a second chamber at pressure P2 from the atmosphere. A rigid link joins the two plates and locates the plates coaxially. The flexibly suspended plates are provided with mechanical stops to limit axial displacement of the plates to a very small range xcex94x.
The first and second circular plates are preferably flat and relatively thick and stiff. First and second flexible suspension members are preferably very thin shells of revolution of a segment of a circle. A thin shell of revolution has a relatively high spring rate when it is clamped at both its inner and outer edges. However, in accordance with the present invention, it has been found that by clamping the outer edge of a thin shell of revolution and joining the inner edge of the shell to the outer edge of a stiff circular plate that the resulting assembly surprisingly has a very low spring rate but can still withstand relatively high pressure differentials without damage.
A flexible plate with very low spring rate would suffer from the same overpressure problems as a thin diaphragm in a capacitance manometer if mechanical deformation were relied upon to provide the opposing force. The present invention uses force balancing means well known in the art to provide the opposing force. In operation, the flexibly suspended plates are constrained to move within a range of axial displacement xcex94x where the manometer is extremely sensitive. When the manometer is not in operation the force balancing assembly rests on a mechanical stop.
When the force balancing assembly rests on a stop, the spring rate of the thin shells increases dramatically compared to the spring rate when the assembly is not against a stop. This large change in spring rate is achieved without the need for elaborate accurately shaped stops for the thin shells as are required in capacitance manometers. Only simple mechanical stops are required. When the force balancing assembly rests against a mechanical stop, the thin flexible shells can resist relatively high pressure differentials without damage if the higher pressure is applied to the concave side. When the force balancing means is in operation the thin flexible shells are in effect clamped at their peripheries and can resist relatively high pressure differentials. However, in operation the force balancing assembly in the present invention has a very low spring rate and can thus measure very low pressure differentials as well as relatively high differentials all in the same manometer. The combination of fixed clamping when not in operation plus pseudo clamping by the force balancing means when in operation provides overpressure protection together with high sensitivity. These advantages have not been achieved simultaneously in the prior art.
The advantages of the present invention are to enable a force balancing manometer which:
1. Has a very wide differential pressure range suitable for clean vacuum processing.
2. Is relatively immune to over pressure.
3. Has negligible hysteresis.
4. Is relatively insensitive to the effects of changes in material properties with temperature and age.
5. Has position sensing and restoring force means located external to the vacuum.
6. Has no electrical circuits inside the vacuum and therefore no need for vacuum feedthroughs.